A principal use for the invention is the production of lead frames for integrated circuits, although many other uses will occur to those skilled in the art, e.g., incandescent lamp leads such as described in U.S. Pat. No. 4,426,598 dated Jan. 17, 1984. With this understanding, the invention will, for convenience, be described with respect to lead frames.
Lead frames serve several purposes in electronic circuit packages, e.g., integrated circuits. They act as the interconnection between the active semiconducting device and the external circuit. They provide mechanical support for the active semiconducting device and provide a path for the dissipation of heat generated within the active semiconducting device.
To meet these requirements, a lead frame material should have suitable strength or stiffness to support the package containing the active semiconducting device (chip), and permit assembly to the external circuit, usually through multiple pins adapted to extend into rather tight fitting sockets without being damaged by bending. The latter consideration becomes increasingly important as the pin count increases and the leads become narrower, and hence weaker. The material of choice must also conform to manufacturing techniques typically used in the fabrication of electronic circuit packages.
A lead frame material must be capable of conducting electricity at low resistance to provide electrical connection between the host circuit and the active semiconducting device, i.e., it should preferably have a high electrical conductivity.
Proper functioning of the semiconductor at rated speed requires that the device be kept cool. This requires that the lead frame material have a high thermal conductivity to enable efficient heat dissipation.
The selected material should be amenable to chemical etching and/or stamping to form the basic shape of lead frame, (See FIG. 1), and have sufficient ductility to withstand forming during manufacture and assembly. Additionally, the lead frame material should be adaptable to electrochemical plating processes.
Lead frames are typically produced from copper alloy strip in very thin section (about 0.002" to about 0.015") because of the high electrical and thermal conductivities of this group of materials. Copper materials generally have adequate stiffness and strength for many current lead frame applications. However, increasingly complex circuits demanding higher pin counts need materials with higher strength and higher electrical and thermal conductivity.
Copper is capable of being strengthened through solid solution addition of other metallic elements. It is well documented that the small additions required to provide substantial strength improvement severely decrease the electrical conductivity when compared to unalloyed copper. Precipitation strengthened copper alloys, while generally having higher electrical conductivities than solid solution strengthened alloys may also have depressed electrical conductivities due to incomplete precipitation of the second phase. Alloys from other metal systems are sometimes used to increase the stiffness or strength of the lead frame in severe applications where a thermal expansion match with that of the encapsulating ceramic is required. Such a change, however, substantially reduces the electrical and thermal conductivity. Dispersion strengthened copper (DSC) however, combines the high strength and high electrical and thermal conductivities in a single material.
Other dispersion strengthened metals such as steel, copper/tin alloys, and the like may also be used in conjunction with this invention. For most purposes however, we prefer to use dispersion strengthened copper having a particle size prior to compaction of less than 20 mesh (U.S. Standard Screen Size) or about 800 microns which material has been internally oxidized prior to its entry into the process hereof.
Internal oxidation of the copper alloy (usually copper/aluminum) is carried out at an elevated temperature, for example from about 1200.degree. F. to about 1800.degree. F., for a period of time sufficient to cause reaction between the solute metal, e.g. aluminum and the oxidant, e.g. cuprious oxide provided in the powder mix.
Although the present invention will be described in connection with dispersion strengthened copper, it is to be understood that the procedures of the present invention are applicable as well to other dispersion strengthened metal powders. Thus, iron, silver, copper/tin (2%) may be dispersion strengthened with refractory metal oxide, such as aluminum oxide(alumina), titanium dioxide, magnesium oxide, silicon dioxide, zirconium oxide, and the like.
Dispersion strengthened copper which can be used with the instant invention can contain from about 0.1% wt. to about 1.1% wt. Al.sub.2 O.sub.3.
Grades of internally oxidized dispersion strengthened copper (DSC) are identified herein by their grade designations as specified by the Copper Development Association: C15715, C15725, C15735, and C15760. These materials are copper based and contain, respectively about 0.3%, 0.5%, 0.7% and 1.1% by weight aluminum oxide. These materials are commercially available under the name "GLIDCOP" which is the registered trademark of SCM Metal Products, Inc., and are identified by SCM numbers "AL-15", "AL-25", "AL-35", and "AL-60" respectively.
The aluminum oxide particle size is exceedingly small, i.e., less than 0.1 micron, for example 30 to 100 Angstroms. The materials can be produced by internal oxidation as described in Nadkarni et al U.S. Pat. No. 3,779,714 dated Dec. 18, 1973 or Nadkarni U.S. Pat. No. 4,315,770 dated Feb. 16, 1982 (now No. Re 31,902 dated May 28, 1984). These patents are incorporated herein by reference thereto.